There are a couple discussions going on at Apple’s discussionboards concerning freezes with the new iMacs that can only be resolved by a hard restart. There have been some links to graphics intensive activities as the cause of the freeze which brings speculation about problems with the graphic card.

Apple is apparently aware of the issue, but has not yet issued a solution. Only time will tell if this is a problem that can be fixed with a firmware update, or whether it will require sending the iMac back to Apple or taking it to a certified technician.